Adsorption of fluoride on synthetic siderite from aqueous solution

The study has investigated the feasibility of using synthetic siderite for F- removal from aqueous solution. Batch experiments were performed to test effects of adsorbent dosage, contact time, initial F concentration, temperature, solution pH, and coexisting anions on F- removal. Results show that the kinetic rate of F- adsorption was high in the first 2 h, and thereafter significantly decreased. The kinetic data was better fitted to the pseudo-second order kinetic model than the pseudo-first order kinetic model. In comparison with Langmuir isotherm, both Freundlich and Redlich-Peterson isotherms better described the adsorption process, which indicates that the multilayer adsorption should be involved in the process of F- removal. Thermodynamic study manifests that F- adsorption on synthetic siderite was spontaneous and exothermic in nature. The synthetic siderite had high adsorption capacity for F- removal, which was up to 1.775 mg/g in the batch with an adsorbent dosage of 5 g/L and an initial F- concentration of 20 mg/L at 25 degrees C. The adsorption was relatively independent on solution pH between 4 and 9. The presence of Cl- and NO3- had less impact on F- adsorption, while PO43- significantly affected F- removal from aqueous solution. Results of X-ray diffraction (XRD) and scanning electron microscopy (SEM) suggest that the high adsorption capacity possibly arose from both coprecipitation of ferric hydroxide with F- and adsorption of F- on the fresh goethite. (C) 2010 Elsevier B.V. All rights reserved.